Of course, some of those old comic book ads were scams for the gullible (like the X-Ray Specs!). But many others were honest and worth the price. I'll always be grateful for the ad I answered as a kid in the 1940s. It was a 'learn to play harmonica by the numbers' kit with harmonica and instructions. After learning to play by the numbers I went on to play any tune by ear. Through the years, the little original harmonica was replaced with bigger more advanced ones, and harmonica became a life-long hobby.
I also answered a Fawcett ad to join the Captain Marvel Club for the price of a comic book (10-cents). I was a little disappointed to get only a short welcome note on a Captain Marvel letterhead and a membership card (with a key to the secret code in every issue of Captain Marvel Comics). That dime would have been better spent on a comic book. Though, I wish now that I still had that letter & card!